Response slowness remains a pervasive characteristic of the behavior of the mentally retarded and the young normal child. Reaction time research has implicated the preparatory processes of response organization and its development. The proposed research will investigate some brain-behavior relations involved in response organization at various levels of motor skill and motor differentiation within different cognitive proficiency levels. The methodology utilizes an analysis of movement-related cortical electrical activity accompanying measures of motor skill development in the mentally retarded, cerebral palzied and normally developing child. This will determine the role of preparatory response organization in motor development and in the impaired behavior of the retarded.